divide error in alarm_forward

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    6531e115b7ab Merge branch 'akpm' (patches from Andrew)
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10c68f6d400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c8970c89a0efbb23
dashboard link: https://syzkaller.appspot.com/bug?extid=9d38bedac9cc77b8ad5e
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+9d38bedac9cc77b8ad5e@xxxxxxxxxxxxxxxxxxxxxxxxx

divide error: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 8629 Comm: syz-executor0 Not tainted 4.20.0-rc6+ #374
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:ktime_divns include/linux/ktime.h:172 [inline]
RIP: 0010:alarm_forward+0xe7/0x180 kernel/time/alarmtimer.c:458
Code: 41 5d 41 5e 41 5f 5d c3 e8 56 44 10 00 31 ff 4c 89 ee e8 7c 45 10 00  
4d 85 ed 78 62 e8 42 44 10 00 48 89 d8 48 8b 4d c8 48 99 <49> f7 fd 48 89  
c3 49 89 c6 48 ba 00 00 00 00 00 fc ff df 49 0f af
RSP: 0018:ffff8881b85b77e0 EFLAGS: 00010046
RAX: 0000000000008a1c RBX: 0000000000008a1c RCX: ffff8881b88e8aa0
RDX: 0000000000000000 RSI: ffffffff816f402e RDI: 0000000000000007
RBP: ffff8881b85b7820 R08: ffff8881b85a8040 R09: 0000000000000004
R10: fffffbfff14a7409 R11: 0000000000000001 R12: ffff8881b88e8a88
R13: 0000000000000000 R14: 0000000000000001 R15: 0000003d6987822d
FS:  00007f31b5e40700(0000) GS:ffff8881daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f31b5e1edb8 CR3: 00000001b9024000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 alarm_forward_now kernel/time/alarmtimer.c:481 [inline]
 alarm_timer_rearm+0xbb/0x150 kernel/time/alarmtimer.c:575
 posixtimer_rearm+0x200/0x3e0 kernel/time/posix-timers.c:321
 dequeue_signal+0x1b0/0x630 kernel/signal.c:680
 signalfd_dequeue fs/signalfd.c:173 [inline]
 signalfd_read+0x295/0x7f0 fs/signalfd.c:226
 __vfs_read+0x117/0x9b0 fs/read_write.c:416
 vfs_read+0x17f/0x3c0 fs/read_write.c:452
 ksys_read+0x101/0x260 fs/read_write.c:578
 __do_sys_read fs/read_write.c:588 [inline]
 __se_sys_read fs/read_write.c:586 [inline]
 __x64_sys_read+0x73/0xb0 fs/read_write.c:586
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457669
Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f31b5e3fc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457669
RDX: 000000000ae3f1a6 RSI: 0000000020000080 RDI: 0000000000000005
RBP: 000000000072bfa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f31b5e406d4
R13: 00000000004c2b0b R14: 00000000004d6608 R15: 00000000ffffffff
Modules linked in:
---[ end trace a0d2cc178e3c2817 ]---
RIP: 0010:ktime_divns include/linux/ktime.h:172 [inline]
RIP: 0010:alarm_forward+0xe7/0x180 kernel/time/alarmtimer.c:458
Code: 41 5d 41 5e 41 5f 5d c3 e8 56 44 10 00 31 ff 4c 89 ee e8 7c 45 10 00  
4d 85 ed 78 62 e8 42 44 10 00 48 89 d8 48 8b 4d c8 48 99 <49> f7 fd 48 89  
c3 49 89 c6 48 ba 00 00 00 00 00 fc ff df 49 0f af
RSP: 0018:ffff8881b85b77e0 EFLAGS: 00010046
RAX: 0000000000008a1c RBX: 0000000000008a1c RCX: ffff8881b88e8aa0
RDX: 0000000000000000 RSI: ffffffff816f402e RDI: 0000000000000007
RBP: ffff8881b85b7820 R08: ffff8881b85a8040 R09: 0000000000000004
R10: fffffbfff14a7409 R11: 0000000000000001 R12: ffff8881b88e8a88
R13: 0000000000000000 R14: 0000000000000001 R15: 0000003d6987822d
FS:  00007f31b5e40700(0000) GS:ffff8881daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f31b5e1edb8 CR3: 00000001b9024000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
syzbot.